Fireplaces generally have a damper to open and close the flue since that when a fire is not burning in the fireplace the damper is closed preventing air from escaping out of the room. It is often difficult to determine whether a damper is open or closed without opening the screen and looking up the flue. Often a person will build a fire without realizing that the damper is closed and thus allowing smoke to escape into the room.
It would be desirable to have an easily attachable indicator which would show whether the damper was open or closed.